Night vision system utilizing a diode laser illumination module and a method related thereto

ABSTRACT

A night vision system  10  is provided for detecting objects at relatively low visible light levels. The system  10  includes an infrared light source  14 . The system  10  further includes a thin sheet optical element  16  extending along a first axis  27  receiving light from the infrared light source  14  and reflecting the light generally in a first direction. Finally, the system  10  includes an infrared camera for receiving the light reflected off objects in the environment and generating a video signal responsive to the received light.

FIELD OF THE INVENTION

[0001] This invention relates to a night vision system for detectingobjects at relatively low visible light levels. In particular, theinvention relates to a night vision system utilizing a thin sheetoptical element to emit infrared light and an infrared camera thatdetects the infrared light reflected off of objects in the environment.

BACKGROUND OF THE INVENTION

[0002] Night vision systems are utilized to allow a user to see objectsat relatively low visible light levels. Further, night vision systemsmay be classified as either passive night vision systems or active nightvision systems. In known passive night vision systems used in automotivevehicles, mid-infrared cameras are used to image objects using theambient infrared light emitted by the objects in the environment.Mid-infrared cameras generally used in automotive applications have arelatively small number of pixels. Accordingly, images formed using suchcameras have a low video resolution and a relatively narrowfield-of-view. Further disadvantages of night vision systems usingmid-infrared cameras include (i) a relatively high manufacturing cost,(ii) generated images that are generally harder for a user to interpretthan images formed using CCD or CMOS cameras, (iii) a potentially poorimage contrast, and (iv) operational restrictions on the location of thecamera resulting in a negative impact to the vehicle styling.

[0003] A known active night vision system utilizes an infrared diodelaser or an incandescent light source and relatively large diameterlenses or reflectors to emit infrared light. The infrared light issubsequently reflected off objects in the environment and is received byan infrared camera. The infrared camera generates a video signalresponsive to the received light. A disadvantage of the known activenight vision system is that the lens or reflector-based illuminatorpackage is relatively large. Accordingly, the known active system cannotbe packaged in a small volume and placed on top of a dash or in a rearview mirror assembly of an automotive vehicle.

[0004] There is thus a need for a night vision system and a methodrelated thereto that minimizes for reduces one or more of theabove-mentioned deficiencies.

SUMMARY OF THE INVENTION

[0005] The present invention provides a night vision system and a methodrelated thereto for detecting objects at relatively low visible lightlevels.

[0006] The night vision system in accordance with the present inventionincludes an infrared light source such as an infrared diode laser. Thesystem further includes a thin sheet optical element extending along afirst axis that receives light from the infrared light source andreflects the light generally in a first direction. Finally, the systemincludes an infrared camera for receiving the light reflected off of anobject in the environment and generating a video signal responsive tothe received light.

[0007] A method for detecting objects in accordance with the presentinvention includes a step of transmitting infrared light to a thin sheetoptical element. The method further includes a step of reflecting thelight from the thin sheet optical element generally in a first directionfrom the thin sheet optical element. The method further includes a stepof receiving the light reflected off of an object in the environment.Finally, the method includes a step of generating a video signalresponsive to the received light.

[0008] The night vision system and the method related thereto inaccordance with the present invention represents a significantimprovement over conventional systems and methods. In particular, thesystem may be packaged in a relatively small volume since the thin sheetoptical element is extremely thin as compared with conventionallenses/illuminators. Accordingly, the inventive system may be located ina greater number of locations in an automotive vehicle. Further, theinventive system is less expensive, has a larger field-of-view, andprovides higher quality images than known passive night vision systems.

[0009] These and other features and advantages of this invention willbecome apparent to one skilled in the art from the following detaileddescription and the accompanying drawings illustrating features of thisinvention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a combination schematic and block diagram of a nightvision system in accordance with the present invention.

[0011]FIG. 2 is a perspective view of a thin sheet optical elementutilized in the night vision system of FIG. 1.

[0012]FIG. 3 is a front view of the thin sheet optical element of FIG.2.

[0013]FIG. 4 is an enlarged fragmentary sectional view of the thin sheetoptical element of FIG. 2 taken along lines 4-4.

[0014]FIG. 5 is an enlarged fragmentary sectional view of the thin sheetoptical element of FIG. 2 taken along lines 5-5.

[0015]FIG. 6 is a graph showing the operational performance of a nightvision system in accordance with the present invention.

[0016] FIGS. 7A-7C are flow charts illustrating a method for detectingobjects in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] Referring now to the drawings wherein like reference numerals areused to identify identical components in the various views, FIG. 1illustrates a night vision system 10 for detecting objects at relativelylow visible light levels. The system 10 may be utilized in a pluralityof applications. For example, the system 10 may be used in an automotivevehicle (not shown) to allow a driver to see objects at night that wouldnot be visible to the naked eye. Further, the system 10 could beutilized as part of a security system wherein a user could see objectsat night in a monitored area. As illustrated, the system 10 includes ahousing 12 which holds the remaining components of the system 10. Itshould be understood, however, that the components of system 10contained in housing 12 could be disposed at different locations whereinthe housing 12 would not be needed. For example, the components of thesystem 10 could be disposed at different operative locations in theautomotive vehicle so that a single housing 12 would be unnecessary.Referring to FIG. 1, the system 10 further includes an infrared lightsource 14, a thin sheet optical element 16, a fiber optic cable 17, aholographic diffuser 18, an infrared camera 20, and an optical bandpassfilter 22. As will be discussed in more detail hereinbelow, the system10 may be used to detect any reflective object, such as an object 24, inoperative proximity to the system 10.

[0018] The housing 12 is provided to enclose and protect the variouscomponents of the system 10. The housing 12 may be constructed from aplurality of materials including metals and plastics.

[0019] The infrared light source 14 is provided to generate infraredlight. Many objects in the environment that are dark in the visiblelight spectrum are brightly illuminated in the infrared light spectrum.Accordingly, it is advantageous to use an infrared light source whentrying to detect objects at night. The light source 14 is conventionalin the art and may comprise an infrared diode laser. As illustrated, thelight source 14 receives an operational voltage V_(cc) from an externalvoltage source (not shown). Further, the light source 14 receive acontrol voltage V_(c) from an external controller (not shown) andgenerate the infrared light responsive to the control voltage V_(c). Inone constructed embodiment, the light source 14 was a Single StripeDiode Laser, Model No. S-81-3000-C-200-H manufactured by Coherent, Inc.of Santa Clara, Calif. As illustrated, the light source 14 may bedisposed in the housing 12. Further, the light source 14 may beconnected to a first end 25 of the fiber optic cable 17 using aconventional light coupler (not shown) as known by those skilled in theart. The second end 26 of fiber optic cable 17 is operatively disposedadjacent to the thin sheet optical element 16. Although the system 10preferably utilizes an infrared light source 14, an alternate embodimentof system 10 may utilize a conventional light source that emits visiblelight (including infrared light), such as a halogen bulb, in lieu of theinfrared diode laser 14.

[0020] The fiber optic cable 17 is utilized to transmit light from thelight source 14 to the thin sheet optical element 16. Because of thehigh brightness (candela per unit area) of the light source 14, cable 17preferably is a relatively small diameter (0.1-1.0 mm) glass fiber. Theuse of a small diameter glass fiber provides several benefits overmonofilament plastic pipes and glass fiber bundles used in non-laserbased remote lighting systems. A small diameter glass fiber is lessbulky than plastic pipes or glass fiber bundles that typically are 10-12mm in diameter. Further, a small diameter glass fiber is significantlyless expensive than monofilament plastic pipes or glass fiber bundles.Still further, a small diameter glass fiber is easier to package,handle, and to install than monofilament plastic pipes or glass fiberbundles.

[0021] The thin sheet optical element 16 is provided to reflect andexpand light (represented by arrows A) generated by the light source 14generally in a first direction from the element 16. In a preferredembodiment, shown in FIGS. 2 and 3, the element 16 comprises a unitarysheet of optical material extending generally along a first axis 27. Theelement 16 preferably has a thickness range from 3-9 mm. It should beunderstood, however, that the element 16 may have a thickness less than3 mm or greater than 9 mm. The element 16 is preferably constructed froma transparent, solid piece of plastic such as polycarbonate and utilizesthe principle of total internal reflection (TIR) to reflect light. TIRis explained in more detail hereinbelow. The element 16 may also beconstructed from other transparent materials such as acrylics. Referringto FIGS. 1, 2 and 3, the element 16 includes a front surface 28, a backsurface 30, a bottom surface 32, a top surface 34, side surfaces 36, 38,and an aspheric lens 40.

[0022] Referring to FIGS. 3 and 4, the bottom surface 32 of element 16defines a first plurality of reflective steps 42 extending generallyalong the axial length of the element 16. Each of the reflective steps42 includes a reflective facet 44 and a tread portion 46. Asillustrated, each tread portion 46 is generally parallel to the axis 27.Each reflective facet 44 is oriented at approximately a 45° anglerelative to the adjacent tread portion 46. It should be understood,however, that the angle of each reflective facet 44 may vary dependingupon the critical angle (discussed further hereinbelow) of therespective facet 44. Further, the reflective facet 44 may have a curvedshape (not shown) to further direct the infrared light in a desireddirection. Still further, the number of reflective steps 42 may vary,and correspondingly, the number of reflective facets 44 may vary.

[0023] The reflective facets 44 utilize the principle of TIR to reflectinfrared light received from the aspheric lens 40 towards the reflectivefacets 50. Total internal reflection of the light occurs when theincident angle θ exceeds the critical angle θ_(c) given by the equationθ_(c)=sin⁻¹(n₁/n₂) wherein n₂ is the index of a refraction of air and n₂is the index of a refraction of the polymeric material used to constructthe element 16. In an alternate embodiment (not shown), the reflectivefacets 44 can be metalized if the light strikes the facets 44 at anangle less than the critical angle.

[0024] Referring to FIGS. 1, 2 and 5, the back surface 30 defines asecond plurality of reflective steps 48 extending generallyperpendicular to the axis 27. Each of the reflective steps 48 includes areflective facet 50 and a tread portion 52. As illustrated, each treadportion 52 is generally perpendicular to the axis 27 and parallel to thefront surface 28. Each reflective facet 50 is oriented at approximatelya 45° angle relative to the adjacent tread portion 52. It should beunderstood, however, that the angle of each reflective facet 50 may varydepending upon the critical angle of the respective facet 50. Further,each reflective facet 50 may have a curved shape (not shown) to furtherdirect the light in a desired direction. Still further, the number ofreflective steps 48 may vary, and correspondingly, the number ofreflective facets 50 may vary. Referring to FIGS. 4 and 5, the facets 50are aligned to receive light reflected from one or more reflectivefacets 44, and, like the facets 44, utilize the principle of TIR. Thefacets 50 reflect the received light through the front surface 28 of theelement 16 as will be described in further detail hereinafter. In analternate embodiment (not shown), the reflective facets 50 can bemetalized if the light from the reflective facets 44 strikes the facets50 at an angle less than the critical angle.

[0025] Referring to FIG. 2, the aspheric lens 40 is provided tocollimate the light exiting the fiber optic cable 17. The axial distancebetween the cable 17 and the lens 40 is chosen such that the lightdiverging from the cable 17 fills the aperature of lens 40. The lens 40is preferably constructed to have a surface of revolution about the axis27 with a circular or hyperbolic cross section. As illustrated, theelement 40 is disposed on the side surface 36 of the element 16 and maybe integral with the element 16. In an alternate embodiment of theelement 16, the lens 40 may comprise a separate lens disposed inoperative proximity to the element 16.

[0026] Referring to FIGS. 1 and 2, the infrared light generated by theinfrared light source 14 is received by the element 16 from the secondend 26 of the fiber optic cable 17. The light being emitted from thesecond end 26 preferably has a spread angle between 20-50°. It should beunderstood, however, that the spread angle may be less than 20° orgreater than 50° depending upon the light characteristics of the cable17. The emitted light enters the element 16 through the aspheric lens 40disposed on the side surface 36 of the element 16. As discussedpreviously, the element 40 collimates the light, which then propagatestoward the reflective facets 44. The reflective facets 50 receive thelight reflected from the facets 44 and further reflect the light throughthe front surface 28 of the element 16 generally in a first direction.

[0027] Referring to FIG. 1, the holographic diffuser 18 is provided toreceive infrared light emitted from the thin sheet optical element 16and to diffuse the light prior to being emitted into the environment.Further, the diffuser 18 in conjunction with the element 16 allows theemitted light to meet a Maximum Permissible Exposure light levelrequiremen as specified in an ANSI standard described in more detailhereinbelow. Still further, the diffuser 18 allows the system 10 tospread the infrared light over the required field-of-view. The diffuser18 is conventional in the art and may be disposed proximate to orintegral with the front surface 28 of the element 16. In a constructedembodiment of the system 10, the diffuser 18 has a vertical spread angleof 5° and a horizontal spread angle of 20°. However, in an alternateembodiment (not shown), the reflective facets 50 of element 16 may beoriented at varying angles with respect to one another to diffuse theinfrared light. Accordingly, in the alternate embodiment, the diffuser18 would not be needed. In another alternate embodiment, an array oflenses (pillow optics) disposed across the exit surface 28 of theelement 16 may be used to diffuse the light instead of the diffuser 18.

[0028] The infrared camera 20 is provided to generate a video signalV_(v) responsive to reflected infrared light received by the camera 20.The camera 20 is conventional in the art and may comprise a CCD cameraor a CMOS camera. In a constructed embodiment of the system 10, the CCDCamera Model No. WAT-902HS manufactured by the Watec AmericanCorporation of Las Vegas, Nev. was utilized. As illustrated in FIG. 1,the infrared camera 20 receives an operational voltage V_(cc) from anexternal voltage supply (not shown). Infrared light emitted from thethin sheet optical element 16 is reflected off of an object 24 in theenvironment and is received by the infrared camera 20. The video signalV_(v) may be applied to a television monitor (not shown) or a heads-updisplay (not shown) in an automotive vehicle to allow a user to see theobject 24.

[0029] The optical bandpass filter 22 is provided to filter the infraredlight reflected from the object 24. In particular, the filter 22 onlyallows light within the infrared light spectrum to be received by thecamera 20. Preferably, the filter 22 allows a maximum transmission oflight at a wavelength equal to the wavelength of light generated by theinfrared light source 14. An advantage of using the filter 22 is thatthe filter 22 prevents blooming (i.e., saturation of the pixel elements)in the camera 20 by visible light emitted from the head lamps (notshown) of other automotive vehicles. The filter 22 is conventional inthe art and is preferably disposed proximate to a receiving lens (notshown) of the camera 20.

[0030] The American National Standards Institute has implemented theAmerican National Standard for Safe Use of Lasers (ANSI Z136.1),hereinafter referred to as the ANSI Standard. The ANSI Standard setsforth guidelines for systems (such as night vision systems) that utilizelasers. In particular, the ANSI Standard specifies that for a givenlaser wavelength, viewing time, illuminator size, and beam divergence,the irradiance (power per unit area) at a given distance from theilluminator (i.e., element 16) must be less than the Maximum PermissibleExposure (MPE) as stated in the ANSI Standard.

[0031] Referring to FIG. 6, a graph illustrates the operationalcharacteristics of the night vision system 10 as compared with the ANSIStandard. In particular, line 53 graphically illustrates the MPE inaccordance with the ANSI Standard. Further, dashed line 55 illustratesthe light irradiance as a function of distance emitted by the thin sheetoptical element 16 when (i) the area of the front surface 28 is 7=7 cm²,(ii) the horizontal and vertical divergence angles of emitted infraredlight are 20° and 5°, respectively, (iii) the wavelength of the infraredlight is 810 nm, and, (iv) the emitted power of the infrared light is 3Watts. As illustrated, the irradiance of the element 16 is less than theMPE at all distances. Accordingly, the night vision system 10 meets theMPE requirements of the ANSI Standard.

[0032] Referring to FIG. 7A, a method 54 for detecting an object 24utilizing a night vision system 10 in accordance with the presentinvention is illustrated. The inventive method 54 may include a step 56of transmitting infrared light to the thin sheet optical element 16. Aspreviously discussed, the infrared light is transmitted to the element16 via the fiber optic cable 17. The method 54 may further include astep 58 of reflecting the light from the thin sheet optical element 16generally in a first direction from the element 16. Referring to FIG.7B, the step 58 may include the substeps 60, 62, and 64. The substep 60involves collimating the light entering the element 16. The substep 62involves reflecting the light from the reflective facets 44 to thereflective facets 50. The substep 64 involves reflecting thepredetermined number of light beams from the reflective facets 50generally in the first direction through the front surface 28 of theelement 16. Referring to FIG. 7A, the method 54 may further include astep 66 of diffusing the light emitted from the element 16. The method54 may further include a step 68 of receiving the light reflected off ofan object 24 in the environment. The object 24 may be one or morereflective objects that are disposed generally in the first directionfrom the element 16. For example, if the system 10 is utilized in anautomotive vehicle (not shown), the object 24 could be a road surface, apedestrian, or an animal. Referring to FIG. 7C, the step 68 may includea substep 72 of filtering the light through an optical band pass filter22. Referring again to FIG. 7A, the method 54 may further include a step70 of generating a video signal V_(v) responsive to the received light.

[0033] The night vision system 10 and the method related thereto inaccordance with the present invention represent a significantimprovement over conventional night vision systems and methods. Inparticular, the system 10 may be packaged in a relatively small packagevolume since the thin sheet optical element 16 is extremely thin ascompared with conventional lenses/illuminators. For example, the system10 may be packaged on the dashboard or inside a rearview mirror assemblyin the interior of the automotive vehicle. Thus, the system 10 providesfor greater design flexibility in automotive vehicles and in otherapplications where a small package volume is required. Another advantageof the system 10 is that the laser light emitted from the element 16meets the ANSI Standard described hereinabove. Further, the inventivesystem 10 is less expensive, has a larger field-of-view, and provideshigher quality images than known passive night vision systems.

[0034] While the invention has been particularly shown and describedwith reference to the preferred embodiment thereof, it is wellunderstood by those skilled in the art that various changes andmodifications can be made in the invention without departing from thespirit and the scope of the invention.

We claim:
 1. A night vision system, comprising: an infrared lightsource; a thin sheet optical element extending along a first axisreceiving light from said infrared light source and reflecting saidlight generally in a first direction; and, an infrared camera forreceiving said light reflected off of an object and generating a videosignal responsive to said received light.
 2. The night vision system ofclaim 1 wherein said infrared light source is an infrared diode laser.3. The night vision system of claim 1 wherein said thin sheet opticalelement includes a first and second plurality of reflective facets, saidfirst plurality of reflective facets receiving said light from saidinfrared light source and reflecting said light to a second plurality ofreflective facets that further reflect said light from said thin sheetoptical element generally in said first direction.
 4. The night visionsystem of claim 3 wherein said thin sheet optical element includes afront surface, a back surface, a bottom surface, and a side surfaceconnected between said front, back, and bottom surfaces, said bottomsurface defining a first plurality of reflective steps extending alongan axial length of said thin sheet optical element, said first pluralityof reflective steps including said first plurality of reflective facets,said back surface defining a second plurality of reflective stepsextending generally perpendicular to said first axis, said secondplurality of reflective steps including said second plurality ofreflective facets aligned to receive said light reflected from saidfirst plurality of reflective facets and to further reflect said lightthrough said front surface.
 5. The night vision system of claim 1wherein said thin sheet optical element is comprised of a polymericmaterial.
 6. The night vision system of claim 1 wherein said thin sheetoptical element further includes an aspheric lens disposed integral withor on said thin sheet optical element, said lens receiving said lightfrom said infrared light source and collimating said light, said lightbeing transmitted from said lens through said thin sheet optical elementto said first plurality of reflective facets.
 7. The night vision systemof claim 1 further comprising a fiber optic cable transmitting saidlight from said infrared light source to said thin sheet opticalelement.
 8. The night vision system of claim 1 further comprising anoptical bandpass filter for filtering said light entering said infraredcamera.
 9. The night vision system of claim 1 further comprising aholographic diffuser or lens array disposed proximate said thin sheetoptical element to diffuse said light being emitted from said thin sheetoptical element.
 10. A night vision system, comprising: an infraredlight source; a thin sheet optical element having an aspheric lens and afirst and second plurality of reflective facets, said aspheric lensbeing integral with or proximate to said thin sheet optical element andreceiving light from said infrared light source and collimating saidlight; said first plurality of reflective facets receiving said lightfrom said aspheric lens and reflecting said light to said secondplurality of reflective facets that further reflect said light generallyin a first direction; and, an infrared camera for receiving said lightreflected off of an object and generating a video signal responsive tosaid received light.
 11. The night vision system of claim 10 whereinsaid infrared light source is an infrared diode laser.
 12. The nightvision system of claim 10 wherein said thin sheet optical elementincludes a front surface, a back surface, a bottom surface, and a sidesurface connected between said front, back, and bottom surfaces, saidbottom surface defining a first plurality of reflective steps extendingalong an axial length of said thin sheet optical element, said firstplurality of reflective steps including said first plurality ofreflective facets, said back surface defining a second plurality ofreflective steps extending generally perpendicular to said first axis,said second plurality of reflective steps including said secondplurality of reflective facets aligned to receive said light reflectedfrom said first plurality of reflective facets and to further reflectsaid light through said front surface.
 13. The night vision system ofclaim 10 wherein said thin sheet optical element is comprised of apolymeric material.
 14. The night vision system of claim 10 furthercomprising a fiber optic cable transmitting said light from saidinfrared light source to said thin sheet optical element.
 15. The nightvision system of claim 9 further comprising an optical bandpass filterfor filtering said light entering said infrared camera.
 16. The nightvision system of claim 10 further comprising a holographic diffuser orlens array disposed proximate said thin sheet optical element to diffusesaid light being emitted from said thin sheet optical element.
 17. Athin sheet optical element for a night vision system, comprising: asheet of optical material extending along a first axis having a frontsurface, a back surface, a bottom surface, and a side surface connectedbetween said front, back, and bottom surfaces, said bottom surfacedefining a first plurality of reflective steps extending along an axiallength of said thin sheet optical element, said first plurality ofreflective steps including a first plurality of reflective facets, saidback surface defining a second plurality of reflective steps extendinggenerally perpendicular to said first axis, said second plurality ofreflective steps including a second plurality of reflective facetsaligned to receive light reflected from said first plurality ofreflective facets and to further reflect said light through said frontsurface.
 18. The thin sheet optical element of claim 17 wherein saidsheet of optical material is comprised of a polymeric material.
 19. Thethin sheet optical element of claim 17 wherein said sheet of opticalmaterial further includes an aspheric lens disposed integral with or onsaid sheet of optical material, said lens collimating light from a lightsource, said light being transmitted from said lens through said sheetof optical material to said first plurality of reflective facets.
 20. Amethod of detecting objects utilizing a night vision system, comprisingthe steps of: transmitting infrared light to a thin sheet opticalelement; reflecting said light from said thin sheet optical elementgenerally in a first direction from said thin sheet optical element;receiving said light reflected off of an object; and, generating a videosignal responsive to said received light.
 21. The method of claim 20wherein said step of reflecting said light, includes the substeps of:collimating said light entering said thin sheet optical element;reflecting said light from a first plurality of reflective facets to asecond plurality of reflective facets; and, reflecting said light fromsaid second plurality of reflective facets generally in said firstdirection.
 22. The method of claim 20 wherein said step of receivingsaid light reflected off of said object includes the substep of:filtering said light through an optical bandpass filter.
 23. The methodof claim 20 further comprising the step of diffusing said lightreflected from said thin sheet optical element.